506 atw Vol. 59 (2014) Issue 8/9 | August/September

NPP Long-term Operation

Forward Alliance – AREVA’s Initiative for NPP’s LTO ProjectsSteffen Bergholz, Benedikt Heinz, Jürgen Rudolph, Erlangen/Germany Perry Quist, Borssele/The Netherlands

Addresses of the Authors:Steffen Bergholz, Benedikt Heinz, Jürgen Rudolph

AREVA GmbHHenry-Dunant-Straße 5091058 Erlangen/Germany

Perry QuistEPZ – Elektriciteits-Productiemaatschappij

Zuid-Nederland N.V.Zeedilk 32

4454 PM Borssele/The Netherlands

High expectations from a growing reactor fleet worldwide

The total electrical production capacity of nuclear power worldwide continues to grow, with almost 371 GWe currently pro-duced by 434 reactors in operation across the globe and 69 reactors under construc-tion [1]. The Fukushima Daiichi accident in March 2011 certainly slowed this growth, but the need for innovative services for ex-isting reactors continues to grow. The latest IAEA scenarios predict an increase in nucle-ar production capacity of between 17 % and 95 % by 2030 [2], and the demands of the global market are growing.

In order to meet CO2 and greenhouse gas emission reduction targets, it is vital to maintain a basic source of nuclear power to compensate for the intermittent nature of solar and wind power contributions to the grid. The fact that 80 % of reactors current-ly in service are more than 20 years old, with half of these over 30 years old [3], means there is high demand for service life extension across the global nuclear fleet.

Thanks to its integrated model, span-ning the complete nuclear cycle from the uranium mines to the recycling of used fu-el, reactor design and maintenance, AREVA supplies services to more than 360 reac-tors of all technology types across the world. To support nuclear utilities in man-

aging LTO projects, AREVA has launched the Forward Alliance initiative, proposing a catalog of the most advanced and effi-cient products, services and solutions.

Forward alliance

Meeting market demand in this area means, first of all, supporting global opera-tors in their own work to extend plants ser-vice life. This involves helping them opti-mize investment in much-needed fleet modernization work, thereby enabling them to take advantage of the value of cur-rent assets in the most effective way. Re-newal of operating licenses, periodic safety reviews, service life extension, etc., differ-ent approaches are taken depending on the country or continent concerned. How-ever, the end goal is the same: to guaran-tee the safe operation of plants over the long term.

These processes correspond to the main steps recommended by the IAEA (Figure 1): • Preliminary studies • Scoping and screening of components• Ageing management reviews by compo-

nent • Implementation of solutions and new

procedures.Responding to the needs also means pro-posing an ageing management approach based on the component: vessel, vessel in-

ternals, pressurizer, steam generator and primary and secondary pumps, to name just the main systems of the nuclear island. The Forward Alliance approach includes a wide range of skills and experience in ma-terial physics and the management of me-chanical fatigue, as well as a precise un-derstanding of the behavior of a wide range of components, whether or not they are AREVA-designed. Solutions including sensors to measure transients, which allow a direct fatigue correlation and the use of historical load evaluation to back up com-ponent ageing management methods, are also available. Another element is helping utilities combat obsolescence via reverse engineering or replacement with new components, a major success factor in ex-tending service life. Modernizing instru-mentation and control systems is without doubt the most relevant example, with digital systems able to deal with obsoles-cence issues in analog systems whose maintenance cost becomes too high. Final-ly, it involves providing nuclear operators with fuel cycle management solutions that are effective until the end of plant service life. This can be done by implementing management solutions for unloading pools, dry storage and spent fuel recycling. These efforts to extend operating licenses are accompanied by a growing demand for assistance in staff training. Managing and anticipating the age and skill distribution of the workforce, as well as guaranteeing the quality of personnel, are vital elements in such an approach. By sharing experi-ence, these projects can be managed in an optimum manner, minimizing investments through wise choices and controlling project costs by maximizing the value of existing assets through service life ex-tension.

Within the ageing management scope, the damaging process caused by fatigue mechanisms is one major concern. It will be escorted by the tightening of fatigue rules due to the consideration of environ-mentally assisted fatigue (EAF) within the fatigue process. The AFC and its major tool FAMOSi offers solutions therefore [4].

Fig. 1. Major phases of an NPP LTO project.

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AREVA Fatigue Concept: General overview

In terms of the nuclear industry, the age-ing management of power plant  compo-nents is nowa days a main issue for all ac-tors: states, regulatory agencies, opera-tors, designers or suppliants. As regards fa-tigue assessment of nuclear components stringent safety standards  imply the consider ation of new parameters  in the framework of the fatigue analysis process: new de sign fatigue curves, consideration of EAF  parameters  and stratifica tion ef-fects. Modern state-of-the-art fatigue mon-itoring approaches gain in importance as part of these ageing management require-ments of NPP components. Consequently, lots of operators have to deal with de-manding security requirements to ensure the safe operation of power plants. The core challenge is the identification and qualified processing of realistic load-time histories.

In this general context AREVA devel-oped the integral approach AFC with new tools and methods in order to live up to op-erators’ expectations. The basic principles of the concept are depicted in Figure 2.

The methods are both valid for the erec-tion and the operation phase. In other words, the first fatigue analyses are based on specified design transients. During op-eration, it is recommended to rely on real-istic operational data as input for the fa-tigue loading. In this sense, the central module is the Fatigue Monitoring Sys-tem FAMOSi as a data logging system for thermal loads like thermal stratification, plug flows or leakage flows. The simple workflow is a two step workflow consist-ing of load determination by local meas-urement (FAMOSi hardware) plus quali-fied fatigue assessment methods (FAMOSi hardware plus specialized tools). These fatigue assessment methods are split up in a three staged fatigue assessment pro-cess [4]:

• Stage 1: Simplified Fatigue Estimation (SFE)Simple estimations of fatigue relevance of real loads for components are based on thermal mechanical considerations using the equation of ideal thermally constrained strains. A basic decision ab-out fatigue relevance (yes/no) for the monitored position is made. In case of fatigue relevance a further evaluation is proposed according to stage 2.

• Stage 2: Fast Fatigue Evaluation (FFE) A design code conforming (cumulative) usage factor CUF is calculated in a highly automated way based on the simplified elasto-plastic fatigue analysis route of relevant design codes.

• Stage 3: Detailed Fatigue Check (DFC)

Fatigue analyses are usually based on a set of model transients from a catalogue as it is shown in Figure 2. Fatigue calcu-lations are usually carried out as simpli-fied elasto-plastic or elasto-plastic ana-lyses based on appropriate material mo-dels.

Note that EAF is considered both in stages 2 and 3 approaches according to the latest international rules and guidelines [5 to 10]. The strain rate and thus time depend-ent consideration of partial EAF Fen,i for each time step i (1s according to the fre-quency of measured data logging) is im-plemented by combining the modified strain rate approach [11] with the rainflow cycle counting algorithm according to Clormann and Seeger [12]. The latest re-quirements regarding fatigue within the German safety standards are equally re-spected and implemented [13]. In fact, us-age factors with and without consideration of EAF are calculated for operational

load  histories considering the require-ments of different design codes: ASME, KTA [13] and expected amendments in RCC-M [14].

Based on real measured thermal loads (FAMOSi local measurements) and super-posed mechanical loads the FFE process allows a highly automated and reliable data processing to evaluate CUF’s of me-chanical components. Calculation and management of results are performed within the software frontend FAMOSi, thus impact of operating cycles on compo-nents in terms of stress and fatigue us-age  can be taken into account in order to  plan optimized decisions relating to the plant operation or maintenance activi-ties. Figure 3 give an example of meas-ured  temperatures, correlating compo-nent stresses and fatigue damage accumu-lation.

The SFE and FFE parts are fully imple-mented and highly automated in the

Fig. 2. General overview of the AREVA Fatigue Concept (AFC).

Fig. 3. Fast Fatigue Evaluation (FFE) gives a direct correlation between temperature and fatigue.

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fatigue assessment software frontend FAMOSi. In the case of increased usage fac-tors the DFC module provides a detailed load case counting and the possible applica-tion of realistic material models within the finite element analysis as well as the fulfill-ment of design code requirements with re-gard of shakedown and ratcheting [15].

Basically, a local acquisition of load da-ta for the follow-up of fatigue trends is rec-ommended. This way it is ensured, that the local loads at the locations of interest with regard to fatigue (e.g. thick walled noz-zles) are captured.

Note that the term “transient” to define the load is used at various occasions. It is useful to exactly differentiate the terms “design transients”, “model transients” and “operational loads”: • Design transients (see Figure 2) are usu-

ally specified in the commissioning phase, i.e. before operation. They are based on plant models and experience and are part of the licensing documents.

• Model transients (see Figure 2) are spec-ified during operation. They are based on operational measurements and/or FAMOSi local measurements.

• Operational loads represent the meas-ured load-time history e.g. based on the FAMOSi software (see Figure 2).

FAMOSi: Hardware and measurement aspects

Thermal load data at the outer surface of the adjacent pipe or directly on elbows are logged by the FAMOSi system by means of thermocouples. The thermocouples are manufactured as measurement sections. In the sense of local fatigue monitoring the measurement sections are located at fa-tigue relevant locations at the outer sur-face of pipes and are based on additional temperature measurement by means of thermocouples. The principle of the meas-urement sections close to the fatigue rele-vant components is shown in Figure 4.

There are different types of measure-ment sections installed at the outer surface of the pipe:

• 7 thermocouples in case of stratifica-tion,

• 2 thermocouples in case of plug flow / thermal shock.

These measurement sections are connect-ed to the FAMOSi Processing Unit (PU) – small boxes located in the containment. The boxes contain all necessary mod-ules  for signal conditioning and digitiz-ing of the thermocouple signals. Further boxes with data acquisition modules are connected via an Ethernet based data bus to the FAMOSi PU. The PU performs the online data reduction, storage and signal analysis functions, as well as the automated fatigue estimation with the SFE.

With the FAMOSi software all data can be displayed in real time and the measure-ment system can be configured. System warnings like detected thermal transients and high fatigue values will be announced at the sequence display within the FAMOSi software.

Local fatigue monitoring: Application example

The FAMOSi system provides fatigue man-agement solutions for all plants in the sense of a generic solution [16]. The real operating thermal load data give real plant transients at the location of the component

of concern (highly stressed/fatigued). These real data deliver real strain rates and thus contribute to eliminate the uncertain-ty in the EAF Fen factor evaluations. Get-ting the real operational data helps the plant engineer to refine the system level/component level design transients.

By way of example, the temperature measurement at an auxiliary spray line is shown in Figure 5. The FFE module is used to calculate the time histories of inner wall temperature and all stresses of the stress tensor. These are the main input data for the fatigue usage factor calculation includ-ing rainflow cycle counting and Fen factor determination.

Furthermore, a comparison of meas-ured temperature for start-up versus allo-cated design transient was carried out. A representative design transient (fluid → heat transfer coefficient → inner wall tem-perature) is compared to the FFE calculat-ed inner wall temperature (see Figure 6). Also shown in Figure 6, the differences in the temperature history between the real-istic (measured) loading and the model transient take a significant influence on the resulting stresses, stress amplitudes and stress ranges. In this example, the stress amplitude resulting from the model transient is about three times bigger than the stress amplitude resulting from the re-alistic operational load (temperature) his-tory with the according effects on calculat-ed CUF. Note that the differences between specified design transients and measured temperature histories are usually even much bigger. A similar comparative study was carried out in [18].

LTO project Borssele

For the lifetime extension project of EPZ’s NPP Borssele (KCB) in the Netherlands, de-tailed fatigue analysis of all major compo-nents were required by the Dutch authori-ty. Also thermal transients that were not considered or known during elaboration of the design transients in the 60ies and 70ies Fig. 4. Local monitoring with FAMOSi measurement sections.

Fig. 5. FAMOSi temperature measured at auxiliary spray line.

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like thermal stratification shall be taken in-to account. As the operational plant instru-mentation is not sufficient for that pur-pose, EPZ decided to install the Fatigue Monitoring System FAMOSi with its local measurement equipment during the plant outage in 2010.

Within the following fuel cycles the FA-MOSi system was used to collect real ther-mal transient data at several class 1 com-ponents. Detailed fatigue analyses were es-tablished and with the real thermal load data conservative assumptions could be re-duced efficiently. Sufficient CUF lower than CUF=1 could be demonstrated for all plant components. In 2014 EPZ received its irrevocable license for 20 more years of  operation until 2034 [18]. Thanks to the strong partnership between EPZ and AREVA in ageing management related top-ics, no components had to be replaced for the lifetime up to 60 years.

Conclusions

LTO is very common in NPP’s world nowa-days. Utilities can realize a high economic value with low effort within LTO projects. AREVA had launched the Forward Alliance initiative which offers different methods and tools to ensure an optimized support of customer’s LTO projects. It consists of a wide spread catalog which covers typical

Fig. 6. Different stress amplitudes between measured and design transient.

III, Division 1, American Society of Mechani-cal Engineers, USA

[7] EPRI Technical Report No. 1025823: Guide-lines for Addressing Environmental Effects in  Fatigue Usage Calculations, Final Re-port, December 2012, EPRI, Palo Alto, CA: 2012.1025823

[8] Stevens, G.L.: Summary of NUREG/CR-6909, Rev. 1 and Suggestions for Future EAF Work, February 12, 2013, ASME Secti-on III Subgroup on Fatigue Strength, Los Angeles, CA

[9] Regulatory Guide 1.207: Guidelines for eva-luating fatigue analyses incorporating the life reduction of metal components due to the effects of the light-water reactor envi-ronment for new reactors, U.S. NRC, March 2007

[10] Chopra, O.; Stevens, G.J.: Effect of LWR Co-olant Environments on the Fatigue Life of Reactor Materials, NUREG/CR-6909 Rev. 1, ANL-12/60, March 2014, Draft Report for Comment; http://pbadupws.nrc.gov/docs/ML1408/ML14087A068.pdf

[11] ASME Working Group (WG) on Environ-mental Fatigue Evaluation Method (2014-02-10)/Subgroup (SG) on Component De-sign (BPV-III) (2014-02-12): Status Report on Item #10-293 (LB#13-2063); Code Case Proposal: Procedure to Determine Strain Rate and Fen

[12] Clormann, U.H.; Seeger, T.: Rainflow – HCM, ein Zählverfahren für Betriebsfestigkeits-nachweise auf werkstoffmechanischer Grundlage, Stahlbau 55 (1986), No. 3, pp. 65-71

[13] Schuler, X.; Herter, K.-H.; Rudolph, J.: Deri-vation of design fatigue curves for austenitic stainless steel grades 1.4541 and 1.4550 within the German Nuclear Safety Standard KTA 3201.2, PVP2013-97138, Proceedings of the ASME 2013 Pressure Vessels & Piping Division Conference, July 14–18, 2013, Pa-ris, France

[14] Métais, T.; Courtin, S.; De Baglion, L.; Genet-te, P.; Gourdin, C.: The Status of the French Methodology Proposal for Environmentally Assisted Fatigue Assessment, PVP2014-28408, Proceedings of the ASME 2014 Pres-sure Vessels & Piping Division Conference, July 20–24, 2014, Anaheim, CA, USA

[15] Kalnins, A.; Rudolph, J.; Willuweit, A.: Using Nonlinear Kinematic Hardening Material Models for Elastic-Plastic Ratcheting Analy-sis, PVP2013-98150, Proceedings of the AS-ME 2013 Pressure Vessels & Piping Division Conference, July 14–18, 2013, Paris, France

[16] Heinz, B.; Wu, D.: AREVA’s Modularized Fati-gue Monitoring for Lifetime Extension and Flexible Plant Operation, PVP2014-28726, Proceedings of the ASME 2014 Pressure Ves-sels & Piping Division Conference, July 20–24, 2014, Anaheim, CA, USA

[17] Rudolph, J.; Bergholz, S.; Hilpert, R.: Detai-led Fatigue Checks Based on a Local Moni-toring Concept, PVP2012-78042, Procee-dings of the ASME 2012 Pressure Vessels & Piping Division Conference, July 15–19, 2012, Toronto, Ontario, Canada

[18] Broy, Y; Linger, M.: EPZ and AREVA – A long-standing partnership for the safe and relia-ble operation of the Dutch Borssele Nuclear Power Plant, atw Vol. 59 (2014), Issue 4 (April)

LTO activities based on AREVA’s experienc-es. The NPP Borssele had realized a license update to 60 years using Forwards Alliance tools.

Acknowledgments

Without the help of several colleagues of AREVA in the United States and Germa-ny, the monitoring system and the meth-ods and tools would not have become reality.

Benoît Jouan is particularly acknowl-edged for preparing the FFE example.

References

[1] IAEA September 2013[2] IAEA-RDS-1/33: Energy, Electricity and

Nuclear Power Estimates for the Period up to 2050; 2013 Edition, August 2013

[3] IAEA/PRIS September 2013[4] http://cdn.intechopen.com/pdfs/33375/

InTechAreva_fatigue_concept_a_three_stage_approach_to_the_fatigue_assess-ment_of_power_plant_components.pdf

[5] Chopra, O.K.; Shack, W.J.: Effect of LWR Co-olant Environments on the Fatigue Life of Re-actor Materials, Report: Argonne National La-boratory, ANL-06/08 (NUREG/CR-6909), Argonne (Illinois, USA), 2007

[6] ASME Code Case N-792, 2010: Fatigue Eva-luations Including Environmental Effects, ASME Boiler & Pressure Vessel Code, Section